Good Afternoon C4 Tech Guru's,

Firstly, I'd like to thank all of you who have contributed to these types of threads over the last 20+ years. This site is truly a library of corvette related information!

My '96 LT4 sprang an exhaust leak a few weeks back and I've spent the last few weekends mending it(and after breaking one of them, replacing the spark plug wires, ugh). I identified the leak as a failed exhaust manifold gasket, replaced the gasket, fired the car back up, and I've still got a rough idle, and gas smell coming from the engine bay(but this time with unburnt fuel spewing out the exhaust pipes at the rear). I pulled out the manifold and removed the heat shield to search for cracks/holes and wasn't able to locate any. I then turned my attention to the exhaust flange gasket("donut").

I purchased GM part number 15170285. From my research, this model number has superseded the old GM model number for this part. I noticed that the new gasket is a bit thicker than the old gasket, but decided to give it a try anyways. I'm having a lot of trouble getting the bolts for the manifold to line up with the holes on the head with the new donut. I've included 2 photos of the new donut compared to the old one that was in the car.

Does anyone have experience with this? Is there some kind of trick to getting the bolts to line up or is the 15170285 not compatible with the LT4 exhaust?

TL;DR: I installed a new exhaust flange donut and now I can't get the header bolted in. Pictures below.

Additional information(if it helps): 1996 LT4, 94K miles. Everything is bone stock.



New exhaust flange gasket next to old. Model number 15170285

Overall height is the same, but the aluminum portion of the donut is thicker on the new donut.



Thanks in Advance!

 

I have never had any luck bolting up any exhaust manifold with the Y pipe attached. I would separate the headers from the Y pipe (or downpipes if running true duals) and install the headers on the engine then install the doughnuts from the bottom.

 

Get a Felpro 61012 and see if it's a closer match to what you removed. I believe it's very likely a much closer match to the original used in the '96 C4. Another option if there's a Workhorse dealer near you is that they may actually still offer the original part # 15713154. How expensive from Workhorse? No idea. Try the Felpro 61012 1st!

There's actually images on eBay where you can 'ZOOM' and inspect the fabrication of the 61012.

If you bought local return your purchase!

The correct seal will install with little effort. Your studs are all in very good condition?

Here's what I'd expect a 61012 Felpro to look like.

 

Yep! I figured that out the hard way. I'm happy to hear it's not just me! I've installed it twice now and based on what I could see and feel the original donut was installed correctly. After I pulled the manifold back out I found some signs of soot coming from a couple of small spots on the donut itself.

 

With the correct/suggested Felpro or original I doubt you'll need to separate any exhaust. Studs and nuts are all 'very good' condition or .........

*** the leakage you observed was the results of incorrect 'donut'.

 

I had considered this, I'll see if I can get one of these local and check it against the original. Perhaps GM part # 15170285 isn't compatible with the stock exhaust manifolds for the LT4.

The old studs were giving me a lot of trouble when removing the old manifold, so I replaced them before trying the 15170285 donut. It goes in and comes out nice and easy now and should provide a more level seal.

Thanks for the valuable input!

Also, pro-tip for anyone in the future searching through threads looking for an answer to their exhaust manifold stud woes: Soak the studs in penetrating oil or a mixture of ATF and Acetone for a night or two(reapply if necessary) and then thread 2 of the exhaust nuts onto each stud and tighten them against eachother until they bind together, then slowly turn the bottom nut counter-clockwise. If you're lucky it'll back the studs out. Don't do what I did and tighten the nuts together with the loop end of your wrench on the bottom nut! You'll have a find time getting the stud off your wrench once it's out of the manifold.

 

A little never seize on the threads will make removal in the future much easier, just don't get any on your hands.
It does not take much for these to leak, if you can find one of the PNs the other posters listed you may have better luck.

 

I just wanted to follow up on this issue.

After spending the last month or so searching for this elusive exhaust leak I decided that I needed to recount every step in this project to make sure I didn't make any stupid errors(spoiler alert - I did).
The other part of this job was replacing the spark plug wires after breaking one. I checked my placement of the plug wires on the Opti after installation, but apparently missed that I switched the the # 6 and #8 plug wires on the passenger side of the car. I had to get low and close to be able to see it clearly. I switched the wires and now the car is running great!

I figured that since I did the wires one at a time that I couldn't have installed them incorrectly. I'll always check my wire placement moving forward, regardless of how sure I am. I'm not even mad, I'm just glad the car's working. It's always something simple!

The gas smell in the engine bay threw me off, so I assumed I must've still had an exhaust leak.

Anyways, thanks again for your help guys!